Can't partition for Boot Camp

I'm trying to install Windows 10 through Boot Camp Assistant on an 27" iMac 5K (late 2015, 1tb Fusion Drive) running Monterey 12.7.6. I have over 300gb free space and want to allocate 192gb to the partition.

I've tried running First Aid, I've checked that there's no 'Free space' partition and deactivated Time Machine updates, but I still get the same error.

I would really appreciate it if somebody could help. Thanks!